The present invention is an optical fiber sensor that utilizes the Bragg effect to enable detection and provision of information relating to a variety of physical phenomena including changes in shear, strain, temperature, and fluid flow. The invention can independently measure physical phenomena simultaneously by use of a single sensing element having two or more particularly-oriented Bragg gratings. The invention has a linear relationship between input and output to facilitate calibration and operation. The invention can provide accurate information affordably in a variety of applications, including monitoring remote and distributed flow in volatile environments such as oil and gas pipes, measuring water flow in hydroelectric power generation environments, measuring shear force impinging on hydrofoils, and monitoring the structural health of buildings and other structures. Because of the ability of the invention to measure shear and temperature independently, the invention has particular utility in distributed and point flow measurement.
Flow measurement techniques presently include thin film anemometry, pressure-sensitive paint, global Doppler velocimetry, and particle image velocimetry. Optical fiber sensors have several inherent advantages over conventional transducers, including, "electrically passive operation, EMI immunity, high sensitivity, and multiplexing capabilities . . . ." Alan D. Kersey et al. "Fiber Grating Sensors," JOURNAL OF LIGHTWAVE TECHNOLOGY Vol. 15 No. 8 1442 at 1442 (August 1997) (hereinafter "Kersey et al."). In addition, fiber grating transducers may be preferred over other types of transducers because they exhibit, "all-fiber geometry, low insertion loss, high return loss or extinction, and potentially low cost [along with] . . . flexibility . . . for achieving desired spectral characteristics." Turan Erdogan, "Fiber Grating Spectra," JOURNAL OF LIGHTWAVE TECHNOLOGY Vol. 15 No. 8 1277 at 1277 (August 1997). However, optical fiber sensors have had difficulty in gaining market acceptance. This may be because "many fiber optic sensors were developed to displace conventional electro-mechanical sensor systems, which are well established, have proven reliability records and manufacturing costs." Id.
Several optical fiber devices have been developed in attempts to overcome these challenges, including sensors that utilize the Bragg effect. A fiber Bragg grating ("FBG") is a group of regular, longitudinally-oriented, finely-spaced, localized alterations in the refractive index of a core of an optical fiber. Fiber Bragg sensors employ FBG's to exploit the Bragg effect within optical fibers and to provide intrafiber, wavelength-specific reflection and transmission. Exposure of an optical fiber with such a grating to physical phenomenon such as strain, stress or temperature changes can cause physical dimensions and optical transmission characteristics of the fiber to be altered. Such alterations can cause changes in that portion of the spectrum of light transmitted through the fiber as well as that portion of the spectrum of light reflected by the grating. Optical fiber sensors that employ a Bragg grating "have an inherent self-referencing capability and are easily multiplexed in a serial fashion along a single fiber." Kersey et al. at 1442. However, "[o]ne of the most significant limitations of FBG sensors is their dual sensitivity to temperature and strain." Kersey et al. at 1449. "On a single measurement of the Bragg wavelength shift, it is impossible to differentiate between the effects of changes in strain and temperature." M.G. Xu et al. "Discrimination between strain and temperature effects using dual-wavelength fibre grating sensors," ELECTRONICS LETTERS Vol. 30 No. 13 1085 at 1085 (Jun. 23, 1994) (hereinafter "Xu et al."). Several complicated methods have been developed in an attempt to differentiate between thermal and strain effects in fiber Bragg grating devices. These include using reference gratings that are "in thermal contact with the structure, but do not respond to local strain changes." Kersey et al. at 1449. Another approach is to "locate two sensor elements which have very different responses to strain . . . and temperature . . . at the same point on the structure." Id. Additionally, two conventional fiber Bragg gratings have been superimposed on the same fiber and slight differences have been detected in the shifts of the two Bragg wavelengths. See Xu et al. passim. Techniques using a fiber optic interferometer and a combination of an FBG and a fiber polarization-rocking filter have also been used to monitor strain and temperature, but such techniques are cumbersome to implement. In addition, each of the foregoing employs unnecessary and expensive complexities in their execution, including multiple sensing elements; complicated equipment, setup, analysis and signal interpretation; non-linear relationships between inputs and outputs; lack of direction sensitivity with regard to strain; and multiple light sources and spectrum analyzers.
Therefore, there is a need for a transducer that utilizes a fiber Bragg grating in a manner that allows the simultaneous measurement of physical phenomena (such as shear force, strain and temperature) in a single sensing element. Additionally, there is a need for such a device to have a highly sensitive, linear response. Furthermore, in addition to the foregoing, there is a need for such a device to have directional sensitivity with regard to strain. Also, there is a need for such a transducer that does not necessarily employ multiple light sources, specific frequency light sources, and/or multiple spectrum analyzers.